Is it possible to customise the layout beyond the typical 'left | Right' or 'Top | bottom' options? What I'd really like to have is to split the right collumn into two viewports. The top one containing a second explorer and the bottom containing the quickview lister. Is it possible?

No, it is not possible with TC. It is not the concept of TC to show more than two windows. But, as you know, there are other filemangers out there that can do that. E.g. Q-Dir can show even 4 windows: http://www.softwareok.de/?seite=Freeware/Q-Dir. If you don*t like the TC concept, why not use one of those others?

First of all, I like TC and have been using it for years. I just never took the time to learn the extent of its features and/or limitations.

Secondly, with respect you don't appear to be a developer therefore to speak of vision / concept is seems presumptuous. Not only that but I think your understanding of the difference between concept and feature is completely misunderstood. The concept, taken directly from the website, is simply: "Total Commander can do much more than Explorer", wheras the dual window is actually a featurenot a concept.

Thirdly, if you'd bother to read my question carefully you'd realise I had already realised that the default TC couldn't and was asking if there was either a plugin to do so or whether the devs / ghisler might consider working on such a feature in future.

Just to be clear, I didn't ask about other software and to write a response that basically says "no, if you don't like TC go away and use something else" to a sensible question seems, at best, idiotic as well as unfriendly.

Now if development is relatively slow in coming and there are features that the core users of TC would rather devs work on fine. Otherwise I don't see the harm in making a suggestion that could add more UI features, especially since TC is pretty lacking in that area these days, what with it's windows 97 look and hundreds of other file managers which are nothing more than a complete rip off from TC that somehow provide a couple of nice extra's.

if you'd bother to read my question carefully you'd realise I had already realised that the default TC couldn't

Well, reading your question carefully, you are asking if it is currently possible:

Is it possible to customise the layout beyond the typical 'left | Right' or 'Top | bottom' options?

a response that basically says "no, if you don't like TC go away and use something else"

The response was no, currently that is not possible, but if you need such a feature, other software might have it, see suggestion. Pointing someone to a competitive software is something not many support forums do. If I were helped this way I'd be thankful and not respond that the answer was

idiotic as well as unfriendly

Impoliteness won't really take you far in this forum, it's full of friendly and polite people here.

I don't see the harm in making a suggestion that could add more UI features

Of course not. It's been suggested several times already, I guess you can find some of such threads in the Suggestions forum and join in if you'd like.